Field of Invention:
The present invention relates generally to the field of the precision measurement of displacement of mechanical assemblies, and more particularly to a displacement transducer utilizing precision electronic measurement of the capacitance of a variable capacitor attached to a movable assembly. The invention is adaptable for use in the measurement of the linear displacement of a hydraulic cylinder piston rod.
It is frequently necessary in equipment of all kinds to know the exact location of one movable part in relation to another part. To provide this information, various position indicating transducers have been devised which usually attach to the exterior of or otherwise sense the position of the movable part.
Initially, many position sensing devices used potentiometers to generate an electrical signal proportional to displacement. The current through or voltage across the potentiometer is used as a measure of linear displacement. Potentiometers come in many forms, both linear and circular, and means are well known in the prior art to link the potentiometers to reflect linear displacement. However, due to their resistive nature, potentiometers are susceptible to inaccuracies caused by temperature variations and wear. Also it has been found difficult to build potentiometers of high resolution to measure large linear displacements.
As machinery is more and more automated with electronic controllers, there is a growing necessity to obtain reliable positional information which can be provided to the controllers. Preferably the information will be presented in digital form. An example of a time modulated position transducer using inductive sensing is Technar Incorporated's 11000 series sensors. However, these sensors are limited to about an inch of linear displacement.
When the movable part whose position needs to be known is a piston rod of a hydraulic cylinder, it is often impractical or undesirable to have an external position sensing transducer. Internally mounted devices have been built and are now commercially available which will provide either a direct indication of the position of a hydraulic cylinder piston or provide such information to a closed loop hydraulic cylinder control system. One currently available example of this type of device is the magnetostrictive transducer of Temposonics, Inc. based on U.S. Pat. No. 3,898,555 by Tellerman. Using this device, a pulse-carrying conductor tube is mounted down the center of the piston rod. A magnet located on the piston head interacts with the magnetic field generated by an electrical pulse sent down the tube to produce a local torsional strain in the magnetostrictive tube which travels the length of the tube at ultrasonic speeds. A measurement of the time between electrical pulse generation and detection of the ultrasonic pulse gives an absolute indication of the position of the piston head. The linear displacement of very long hydraulic cylinders may be measured by this type of device However, the repetition rate at which measurements can be determined is dependent on the displacement being measured. The above mentioned device requires a complex electro-mechanical assembly resulting in significant costs for the device which may well be in excess of the cost of the hydraulic cylinder itself.
The SH25 hydraulic cylinder displacement transducer made by Hvilsted a/s gives an absolute position indication using an inductive sensing element built into the piston rod. However, it is limited to a 250 cm stroke length. The present invention may be mounted inside a hydraulic cylinder or externally to an actuation arm and accurately determines the absolute position of a hydraulic piston using relatively simple mechanical and electronic components and can, therefore, be manufactured at significantly reduced costs. In addition, the output virtually instantaneously reflects the displacement no matter how long or short a displacement is being measured.
Capacitance measurements have been used in the prior art to measure changes in linear dimensions. Patent Nos. 3,729,985 by Sikorra and 4,197,753 by Harting and Egger disclose strain gauges in which linear displacement is detected and measured by a differential capacitance detector. In these patents, a plate common to two capacitors is mounted either on a fixed or movable member while the two non-common plates are mounted on the other member. In Sikorra, the capacitors form half of a bridge circuit. Movement is measured by measuring the unbalanced output of the bridge circuit as the capacitor values change. Harting and Egger drive the differential capacitors with opposite ends of a center tapped secondary winding of a carrier transformer. The varying output signal from the movable plate is amplified and further processed by means known in the art.
Similarly patent No. 4,054,049 by Egger discloses a capacitance based linear displacement sensor for use in a thermal extensometer which utilizes a differential capacitance measured by a bridge circuit. It should be noted that the above cited patents utilize capacitance measurement to detect very small changes in linear dimension induced by thermal or strain distortions.
The sensitivity of capacitance changes has also been utilized in patent No. 3,365,946 by Hall. In this patent, a moveable central dividing plate between two unequal gas filled enclosures forms a common plate of two capacitors. The movement or displacement of the central plate changes the capacitance of both capacitors in an opposite direction, thus providing a differential capacitance measurement of linear displacement. Again, a bridge circuit is used to detect the changes in capacitance. In all the above cited patents, differential capacitance measurements were employed to measure very slight changes in linear dimension Previous to the present invention, the measurement of the capacitance of a single variable coaxial capacitor to measure precisely relatively large linear displacements has not been known.